fix: use UTF-16 length for Telegram stream consumer message splitting

The stream consumer measured message length using Python's len() (Unicode
code points), but Telegram's actual limit is in UTF-16 code units. This
caused messages with supplementary characters (emoji, CJK, etc.) to exceed
Telegram's 4096-character limit, resulting in truncated messages with
formatting artifacts.

Changes:
- Add message_len_fn property to BasePlatformAdapter (defaults to len)
- Override in TelegramAdapter to return utf16_len
- Stream consumer uses adapter.message_len_fn for:
  - safe_limit calculation
  - overflow detection
  - truncate_message calls
  - split point calculation (via _custom_unit_to_cp)
  - fallback final send chunking

Fixes truncated messages with black square artifacts on Telegram when
the model generates responses containing multi-byte Unicode characters.

gateway/platforms/base.py

@@ -1311,6 +1311,15 @@ class BasePlatformAdapter(ABC):
         # _keep_typing skips send_typing when the chat_id is in this set.
         self._typing_paused: set = set()
 
+    @property
+    def message_len_fn(self) -> Callable[[str], int]:
+        """Return the length function for measuring message size on this platform.
+
+        Override in adapters whose platform counts characters differently from
+        Python ``len`` (e.g. Telegram counts UTF-16 code units).
+        """
+        return len
+
     @property
     def has_fatal_error(self) -> bool:
         return self._fatal_error_message is not None

gateway/platforms/telegram.py

@@ -283,6 +283,11 @@ class TelegramAdapter(BasePlatformAdapter):
     MEDIA_GROUP_WAIT_SECONDS = 0.8
     _GENERAL_TOPIC_THREAD_ID = "1"
 
+    @property
+    def message_len_fn(self):
+        """Telegram measures message length in UTF-16 code units."""
+        return utf16_len
+
     def __init__(self, config: PlatformConfig):
         super().__init__(config, Platform.TELEGRAM)
         self._app: Optional[Application] = None

gateway/stream_consumer.py

@@ -21,7 +21,10 @@ import queue
 import re
 import time
 from dataclasses import dataclass
-from typing import Any, Optional
+from typing import Any, Callable, Optional
+
+from gateway.platforms.base import BasePlatformAdapter as _BasePlatformAdapter
+from gateway.platforms.base import _custom_unit_to_cp
 
 logger = logging.getLogger("gateway.stream_consumer")
 
@@ -301,9 +304,18 @@ class GatewayStreamConsumer:
 
     async def run(self) -> None:
         """Async task that drains the queue and edits the platform message."""
-        # Platform message length limit — leave room for cursor + formatting
+        # Platform message length limit — leave room for cursor + formatting.
+        # Use the adapter's length function (e.g. utf16_len for Telegram) so
+        # overflow detection matches what the platform actually enforces.
+        # Gate on isinstance(BasePlatformAdapter) so test MagicMocks (whose
+        # auto-attributes return mock objects, not callables) fall back to len.
+        _len_fn: "Callable[[str], int]" = (
+            self.adapter.message_len_fn
+            if isinstance(self.adapter, _BasePlatformAdapter)
+            else len
+        )
         _raw_limit = getattr(self.adapter, "MAX_MESSAGE_LENGTH", 4096)
-        _safe_limit = max(500, _raw_limit - len(self.cfg.cursor) - 100)
+        _safe_limit = max(500, _raw_limit - _len_fn(self.cfg.cursor) - 100)
 
         try:
             while True:
@@ -345,6 +357,10 @@ class GatewayStreamConsumer:
                     should_edit = should_edit or (
                         (elapsed >= self._current_edit_interval
                             and self._accumulated)
+                        # buffer_threshold is intentionally codepoint-based:
+                        # it's a debounce heuristic ("send updates roughly
+                        # every N visible characters"), not a platform-limit
+                        # check. _len_fn is reserved for overflow detection.
                         or len(self._accumulated) >= self.cfg.buffer_threshold
                     )
 
@@ -353,7 +369,7 @@ class GatewayStreamConsumer:
                     # Split overflow: if accumulated text exceeds the platform
                     # limit, split into properly sized chunks.
                     if (
-                        len(self._accumulated) > _safe_limit
+                        _len_fn(self._accumulated) > _safe_limit
                         and self._message_id is None
                     ):
                         # No existing message to edit (first message or after a
@@ -362,7 +378,7 @@ class GatewayStreamConsumer:
                         # proper word/code-fence boundaries and chunk
                         # indicators like "(1/2)".
                         chunks = self.adapter.truncate_message(
-                            self._accumulated, _safe_limit
+                            self._accumulated, _safe_limit, len_fn=_len_fn,
                         )
                         chunks_delivered = False
                         reply_to = self._message_id or self._initial_reply_to_id
@@ -389,11 +405,14 @@ class GatewayStreamConsumer:
                     # Existing message: edit it with the first chunk, then
                     # start a new message for the overflow remainder.
                     while (
-                        len(self._accumulated) > _safe_limit
+                        _len_fn(self._accumulated) > _safe_limit
                         and self._message_id is not None
                         and self._edit_supported
                     ):
-                        split_at = self._accumulated.rfind("\n", 0, _safe_limit)
+                        _cp_budget = _custom_unit_to_cp(
+                            self._accumulated, _safe_limit, _len_fn,
+                        )
+                        split_at = self._accumulated.rfind("\n", 0, _cp_budget)
                         if split_at < _safe_limit // 2:
                             split_at = _safe_limit
                         chunk = self._accumulated[:split_at]
@@ -584,14 +603,18 @@ class GatewayStreamConsumer:
         return final_text
 
     @staticmethod
-    def _split_text_chunks(text: str, limit: int) -> list[str]:
+    def _split_text_chunks(
+        text: str, limit: int,
+        len_fn: "Callable[[str], int]" = len,
+    ) -> list[str]:
         """Split text into reasonably sized chunks for fallback sends."""
-        if len(text) <= limit:
+        if len_fn(text) <= limit:
             return [text]
         chunks: list[str] = []
         remaining = text
-        while len(remaining) > limit:
-            split_at = remaining.rfind("\n", 0, limit)
+        while len_fn(remaining) > limit:
+            _cp_budget = _custom_unit_to_cp(remaining, limit, len_fn)
+            split_at = remaining.rfind("\n", 0, _cp_budget)
             if split_at < limit // 2:
                 split_at = limit
             chunks.append(remaining[:split_at])
@@ -647,8 +670,13 @@ class GatewayStreamConsumer:
                 return
 
         raw_limit = getattr(self.adapter, "MAX_MESSAGE_LENGTH", 4096)
+        _len_fn: "Callable[[str], int]" = (
+            self.adapter.message_len_fn
+            if isinstance(self.adapter, _BasePlatformAdapter)
+            else len
+        )
         safe_limit = max(500, raw_limit - 100)
-        chunks = self._split_text_chunks(continuation, safe_limit)
+        chunks = self._split_text_chunks(continuation, safe_limit, len_fn=_len_fn)
 
         stale_message_id = self._message_id  # partial message to clean up
         last_message_id: Optional[str] = None